Throughout this application various patents and published patent applications are referred to by an identifying citation. The disclosures of the patents and published patent applications referred to in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
An electroactive material that has been fabricated into a structure for use in a battery is referred to as an electrode. Of a pair of electrodes used in a battery, herein referred to as a chemical source of electrical energy, the electrode on the side having a higher electrochemical potential is referred to as the positive electrode, or the cathode, while the electrode on the side having a lower electrochemical potential is referred to as the negative electrode, or the anode.
An electrochemically active material used in the cathode or positive electrode is referred to hereinafter as a cathode active material. An electrochemically active material used in the anode or negative electrode is hereinafter referred to as an anode active material. Multi-component compositions possessing electrochemical activity and comprising an electrochemically active material and optional electron conductive additive and binder, as well as other optional additives, are referred to hereinafter as electrode compositions. A chemical source of electrical energy or battery comprising a cathode with the cathode active material in an oxidized state and an anode with the anode active material in a reduced state is referred to as being in a charged state. Accordingly, a chemical source of electrical energy comprising a cathode with the cathode active material in a reduced state, and an anode with the anode active material in an oxidized state, is referred to as being in a discharged state.
A lithium, sodium or other alkali metal salt or mixture of such salts dissolved in a solvent or mixture of solvents so as to maintain conductivity in the solution is referred to hereinafter as a supporting salt.
There is a wide variety of electroactive materials that may be utilized in the cathode active layers of chemical sources of electrical energy. For example, a number of these are described in U.S. Pat. No. 5,919,587 to Mukherjee et al. These electroactive materials vary widely in their specific densities (g/cm3) and in their specific capacities (mAh/g) so the desired volumetric densities in mg/cm3 of the electroactive material in the cathode active layer correspondingly vary over a wide range. Lithium and sulphur are highly desirable as the electrochemically active materials for the anode and cathode, respectively, of chemical sources of electrical energy because they provide nearly the highest energy density possible on a weight or volume basis of any of the known combinations of active materials. To obtain high energy densities, the lithium may be present as the pure metal, in an alloy, or in an intercalated form, and the sulphur may be present as elemental sulphur or as a component in an organic or inorganic material with high sulphur content, preferably above 75 weight percent sulphur. For example, in combination with a lithium anode, elemental sulphur has a specific capacity of 1680 mAh/g. This high specific capacity is particularly desirable for applications such as portable electronic devices and electric vehicles, where low weight of the battery is important.
Solutions of lithium salts with large anions in individual aprotic dipole solvents and their mixtures are widely used as electrolytes in lithium and lithium-ion rechargeable batteries. The main requirements of these electrolytes are:                high conductivity;        capability to stay in a liquid or gel (for gel electrolytes) state over a wide temperature region;        high stability against electrode active materials;        chemical and electrochemical stability (wide electrochemical stability region);        fire and explosion safety;        nontoxicity.        
High conductivity over a wide temperature range is the main of the above mentioned requirements. The electrolyte conductivity is determined by the physical and chemical properties of the solvents and salts. To obtain high conductivity, it is preferred to use solvents having high donor characteristics, a high dielectric constant, and low viscosity, thus providing a high dielectric dissociation degree for the lithium salts. Lithium salts with large anions are preferably used since these have a high dissociation ability.
The conductivity of the salt solutions is determined by their concentration. With an increase of salt concentration, the conductivity at first increases, then reaches a maximum and finally decreases. The salt concentration is usually chosen to provide maximum conductivity of the resulting electrolyte [Lithium batteries: Science and Technology; Gholam-Abbas Nazri and Gianfranco Pistoia (Eds.); Kluwer Academic; published 2004; pp. 509-573].
Solutions of one or several lithium salts in individual solvents or their mixtures are also used as electrolytes in lithium-sulphur batteries [U.S. Pat. No. 6,030,720, Chu et al]. The choice of solvents is the main concern when designing electrolytes for lithium-sulphur batteries because the nature (the physical and chemical properties) of the solvents has the principal influence on the battery properties.
The electrolyte salts that are used in the main prior art lithium and lithium-ion batteries can be used as supporting salts in lithium-sulphur batteries. As a rule, prior art patent disclosures of which the present applicant is aware do not provide recommendations for specific preferable salt concentrations, but instead give a very wide range of possible concentrations.
The nearest closest prior art to the present invention is currently believed to be described in U.S. Pat. No. 6,613,480 to Hwang, et al. The text of the patent discloses the information that electrolyte salts for lithium-sulphur batteries can be chosen from a list containing: lithium hexafluorophosphate (LiPF6), lithium hexafluorarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium sulfonylimid trifluoromethane (LiN(CF3SO2)2)) and lithium trifluorosulfonate (CF3SO3Li). The electrolyte salt concentration should be taken from the range of 0.5 to 2.0M.
High conductivity over a wide temperature range (together with electrochemical stability) is the main requirement of the electrolyte compositions used in lithium and lithium-ion batteries with traditional hard cathode active materials. The choice of the electrolyte composition for lithium-sulphur batteries is much harder because the sulphur may dissolve in the electrolyte solvents and react with their components, with this having a major influence on the battery properties.
Despite the numerous electrolyte solvents and electrolyte salts proposed for use in rechargeable cells, there remains a need for improved non-aqueous electrolyte compositions that provide beneficial effects during the useful life of the chemical sources of electric energy comprising sulphur-based positive electrode active material.